Crosslinked,heat-shrinkable polyethylene film composition

ABSTRACT

A HEAT-SHRINKABLE, HEAT-SEALABLE FILM PREPARED BY CROSSLINKING AN ORIENTED FILM OF AN BLEND OF LOW-DENSITY AND HIGH-DENSITY POLYETHYLENES CONTAINING A MINOR AMOUNT OF AN ETHYLENE, PROPYLENE, NON-CONJUGATED DIENE TERPOLYMER. THE CROSSLINKING IS ACCOMPLISHED WITH SULFUR MONOCHLORIDE.

United States Patent Office 3,592,881 CROSSLINKED, HEAT-SHRINKABLE POLY-ETHYLENE FILM COMPOSITION George Joseph Ostapchenko, Williamsville,N.Y., assignor to Eg.ldu Pont de Nemours and Company, Wilmington, e NoDrawing. Filed June 16, 1969, Ser. No. 833,767 Int. Cl. (30% 29/12 US.Cl. 260897 4 Claims ABSTRACT OF THE DISCLOSURE A heat-shrinkable,heat-scalable film prepared by crosslinking an oriented film of a blendof low-density and high-density polyethylenes containing a minor amountof an ethylene, propylene, non-conjugated diene terpolymer. Thecrosslinking is accomplished with sulfur monochloride.

BACKGROUND OF THE INVENTION This invention relates to polyethylene filmand, more particularly, to heat-shrinkable polyethylene film.

Because of its many excellent properties, polyethylene film has beenused extensively for wrapping and packaging. In such uses, it is oftendesirable for the film to be heat-shrinkable so as to facilitate thepreparation of a skin-tight package and, in addition, be heat-scalableso as to be usable on automated packaging equipment. It is difficult toprovide a film that satisfies both of these criteria, and previousattempts to do so have often resulted in films which requiredtemperatures greatly in excess of 100 C. for heat shrinking or had arelatively low shrink force. Unfortunately, however, such temperaturesare very near the zero strength temperature of ordinary polyethylene,i.e., the temperature at which the polyethylene film has a tensilestrength which is nearly zero.

One approach to the problem of producing a heatshrinkable film havinggood shrinking characteristics and high shrink force has been to treatthe film with high energy irradiation in order to permit hot-stretchorienting as in Rainer et al., US. Pats. 2,877,500 and 3,144,399.Equipment for carrying out such a treatment is expensive. A furthercomplication is that the film so produced is not amenable to beingreprocessed because the irradiative crosslinking precedes the stretchingand trimming operation, thus militating against efiicient scrap and trimrecovery.

Another approach to the production of a heat-shrinkable film is thatinvolving the extrusion and stretching of a blend of high-density andlow-density ethylene polymer, as described in Golike, US. Pat.3,299,194. Such a system has the advantage that there is no need toirradiate the extruded film prior to orientation as is the case with thetype of product described in US. Pat. 3,144,399. However,heat-shrinkable film made from the blend of resins has the shortcomingof a relatively narrow heatsealing range. The ability to be heat sealedover a relatively broad temperature range becomes highly important whenthe film is to be applied to the wrapping of articles on automaticpackaging machinery. An approach to improving the film of the blend ofpolymers in this respect has been to irradiate the oriented film eitherby ultraviolet irradiation or by high energy irradiation for a timesufficient to raise the zero strength temperature of the film. Again,especially in the case of the high energy irradiation, this approachentails the procurement of expensive irradiation equipment and in somesituations it might be desirable not to have to impose a subsequentirradiation step whether it be by ultraviolet or high energyirradiation.

3,592,881 Patented July 13, 1971 As can be seen from the foregoing, itwould be quite desirable to provide a heat-shrinkable polyethylene filmhaving a broad heat-sealing range which is suitable for use on automaticpackaging equipment and which can be prepared by a relatively simplemanufacturing process. Substantial realization of these goals isprovided by the instant invention.

SUMMARY OF THE INVENTION A crosslinked, oriented film of a blend of: (A)70 to weight percent of a low-density polyethylene, (B) 15 to 30 weightpercent of a high-density polyethylene, and (C) 5 to 15 weight percentof an ethylene, propylene, nonconjugated diene terpolymer, saidpercentages being based on the total Weight of (A) and (B); having alatent shrinkage of at least about 15% in each direction in the plane ofthe film at a temperature of C.

This invention further comprises a process for the preparation of thesefilms by forming the blend components into a self-supporting film,heating the film to a temperature of about 90 C. to C. and stretchingthe film at least 5 times (5 X) in each of two mutually perpendiculardirections, cooling the film while maintaining the film substantially atits stretched dimension, and thereafter subjecting the film to theaction of sulfur monochloride to a degree sufiicient to raise the zerostrength temperature of the film to at least about C.

DETAILED DESCRIPTION OF THE INVENTION The term low-density polyethyleneas used in this description means either homopolymers of ethylene orethylene copolymers having a density of from about 0.91 to 0.93 gram/cc.at 25 C. Copolymers of ethylene are defined as those containing a minoramount of another olefinically unsaturated monomer copolymerizable withethylene, e.g., vinyl acetate, methyl methacrylate, ethyl acrylate,styrene, vinyl, vinyl methyl ether, diisobutylene, methacrylic acid andacrylonitrile. Such low-density polymers are ordinarily produced by freeradical catalysis and are typically referred to as branchedchainpolyethylenes. Golike, US. Pat. 3,299,194, contains further informationabout such polymers.

The term high-density polyethylene is defined herein as homopolymers ofethylene or copolymers of ehtylene with another monomer having a densityof from about 0.94 to 0.98 gram/ cc. at 25 C. The copolymers refer topolymers of ethylene with a minor amount of another olefinicallyunsaturated monomer copolymerizable therewith, e.g., propylene,l-butene, l-pentene, l-hexene, 1- heptene, l-octene, l-decene, as wellas mixtures of these monomers. The high-density polymers are typicallyproduced by coordination-type catalysis and are commonly referred to aslinear polyethylene. More details about such polymers are contained inGolike, US. Pat. 3,299,194.

The term ethylene, propylene, non-conjugated diene terpolymer is definedas an elastomeric terpolymer containing from about 20 to 75 weightpercent ethylene and about 1 to 15 mole percent of the diene. The dieneis defined as a non-conjugated alkadiene containing from 5 to about 22carbon atoms and having at least one terminal double bond. Typicaldienes are 1,4-hexadiene (which is preferred) and the several otherdienes listed in Gresham et al., US Pat. 2,933,480. Other terpolymers ofethylene and propylene are those wherein the third monomeric unit is oneof the following: dicyclopentadiene, an alkenyl norbornene such asethenyl norbornene, an alkylene norbornene such as methylene norbornene,an alkylidene norbornene such as ethylidene norbornene, and1,5-cyclooctadiene. The terpolymers as defined herein have an intrinsicviscosity of from about 0.5 to 5 in benzene at 25 C. Details concerningthe preparation of such terpolymers are available in the following U.S.patents: Gresham 3 et al., 2,933,480; Tarney, 3,000,866; Gladding etal., 3,063,963; 3,093,620; and 3,093,621; and Natta et al., 3,260,708.

The blending of the components, the extrusion and orientation of thefilm can be conducted substantially as described in Golike, U.S. Pat.3,299,194, and Goldman et al., U.S. Pat. 3,149,912. The sulfurmonochloride (S Cl may be incorporated into the film in any suitableway. A particularly convenient method involves immersing the film inliquid sulfur monochloride or in a solution of sulfur monochloride in anappropriate solvent such as carbon disulfide, ether, benzene, ortoluene, and thereafter removing the solvent and any residual by-productfrom the film by gentle heating.

Preferably, the blend of the highand low-density ethylene polymersshould contain from about 70% to about 80% by weight of the low-densitypolymer and from about 20% to about 30% f the high-density polymer. Itis preferred that the high-density component comprise a copolymer ofethylene with a minor amount of another a-olefin, in particular, onehaving from 4 to carbon atoms. Preferably, the high density componentshould have a melt index of from about 0.4 to 1.5 and a density of fromabout 0.950 to about 0.957 gram/cc. Preferably, the low-densitycomponent should have a density of from about 0.91 to about 0.92gram/cc. and a melt index of from about 1.5 to 8. It is furtherpreferred that the exposure of the film to the crosslinking agent besuch that the zero strength temperature of the treated film is at least190 C.

The invention will now be described in connection with a specificexample thereof wherein parts and percentages are by weight unlessotherwise specified.

EXAMPLE To illustrate the Workings of this invention, 180 parts of amelt blend comprising 75% of a branched-chain polyethylene (ethylenehomopolymer) resin having a melt index of 4.1 and a density of 0.913gram/cc. at C, and 25% of a linear polyethylene resin (an ethylene/l-octene copolymer) having a melt index of 0.45 and a density of 0.956gram/cc. at 25 C. with 20 parts of a terpolymer comprising 72% ofethylene, 24% of propylene and 4% of 1,4-hexadiene are melt-blended in ascrew extruder at 200 C. and then formed into a film. The melt blendalso contains 250 parts per million of an anti-oxidant (2,6-di-tbutyl-4-methylphenol), 800 parts per million of mixed amide(approximately 533 parts of oleamide and 267 parts of stearamide) and500 parts per million of silica. This resin is extruded through anapparatus such as is described in Goldman et al., U.S. Pat. 3,141,912.Conditions of extrusion and orientation are:

Melt temperature: 200 C. Die opening: mils Cast tube thickness: 20 milsExtent of stretch:

5 X (machine direction) 5 X (transverse direction) Orientationtemperature: 115 C.

The resulting oriented film is then immersed in liquid sulfurmonochloride held at room temperature, withdrawn immediately, excesssulfur monochloride is removed by wiping the surface, and the film isthereafter dried. An evaluation of the test film along with a suitablecontrol film which is made exactly as described for the test film exceptthat it is not crosslinked with sulfur monochloride (and therefore isnot within the scope of this invention) is shown below in tabular form.

Uncrosslinkvd The addition of the terpolymer is found to improve thehaze, shrinkage, gloss and pneumatic impact and the resulting film madefrom the blend can be crosslinked to give improved burn-throughresistance.

As many widely different embodiments of this invention may be madewithout departing from the spirit and scope thereof, it is to beunderstood that this invention is not limited to the specificembodiments thereof except as defined in the appended claims, and allchanges which come within the meaning and range of equivalence areintended to be embraced therein.

What is claimed is:

1. A crosslinked, oriented film of a blend of: (A) to weight percent ofa low-density polyethylene, (B) 15 to 30 weight percent of ahigh-density polyethylene, and (C) 5 to 15 weight percent of anethylene, propylene, non-conjugated diene terpolymer, said percentagesbeing based on the total weight of (A) and (B); having a latentshrinkage of at least about 15% in each direction in the plane of thefilm at a temperature of C. and a zero strength temperature of at leastabout C.

2. A film as defined in claim 1 wherein said low-density polyethylene isan ethylene homopolymer, said highdensity polyethylene is an ethylenecopolymer with a minor amount of another a-olefin having from 4 to 10carbon atoms, and said non-jugated diene is 1,4-hexadiene.

3. A film as defined in claim 2 wherein the percentage of saidlow-density polyethylene is from about 70 percent to 80 percent and thepercentage of said high-density polyethylene is from about 20 percent to30 percent.

4. A film as defined in claim 2 wherein said a-olefin is l-octene.

References Cited UNITED STATES PATENTS 3,299,194 l/1967 Golike 2642103,261,889 7/1966 Vant Wout 260897 3,407,253 10/1968 Yoshirnura et al264289 MURRAY TILLMAN, Primary Examiner C. I. SECCURO, AssistantExaminer U.S. Cl. X.R.

